Comp151
Example: Definition
/* reverse_print.cpp */
#include <iostream> #include <string>
using namespace std;
int global_var = 23; // global variable definition
void reverse_print(const char* s) // function definition {
for (int j = strlen(s) - 1; j >= 0; --j) cout << s[j];
cout << endl; }
Example: Declaration
/* use_reverse_print.cpp */
#include <iostream>
using namespace std;
extern int global_var; // external variable declaration
extern void reverse_print(const char* s); // external function declaration
int main(int argc, const char* argv[])
{
float local_var; // local variable definition
local_var = 987.654;
cout << ''global var = '' << global_var << endl; cout << ''local var = '' << local_var << endl; cout << ''input string backwards = '';
reverse_print(argv[1]); }
Definition
• A definition introduces a variable's or a function's name
and type.
• A variable definition reserves a number of bytes of
memory for the variable.
• A function definition generates code for the function.
• In both cases, definitions cause the compiler to allocate
memory to store the variable or function code.
•
An object must be defined exactly once in a program.*
Declaration
• The declaration of a variable (or function) announces that the variable (or function) exists and is defined somewhere else (in the same file, or in a different file). The connection is made when the object files are linked.
• A variable declaration consists of the variable's name and its type preceded by the keyword extern.
• A function declaration consists of the function prototype (without the function body) preceded by the keyword extern.
• A (forward) class declaration consists of the class name (without the class body) preceded by the keyword class.
• A declaration does not generate code, and does not reserve memory. • There can be any number of declarations for the same object name in a
program (as long as they’re consistent with each other).
• If a declaration is used in a file different from that with the definition of the object, the linker will insert the real memory address of the object instead of the symbolic name.
• In C++, a variable must be defined or declared to the program before it is used.
A word on class definitions
• A class definition defines a type.
• Note that merely defining a type or class does not
generate code. (The code for any member function of
the class is not generated until the compiler sees an
individual member function definition.)
• In this sense, class definitions are unlike variable and
function definitions, which cause the compiler to allocate
memory to store the variable or function code.
• But class definitions are still like variable and function
definitions in the sense that a class must be defined
exactly once in a program.
Advantages of Header Files
• In general, a header file provides a centralized location for:
– external object declarations
– function declarations & member function declarations
– type definitions
– class definitions (but not member function definitions, except inline)
– inline function definitions & inline member function definitions
• The advantages are:
– 1. By including the header files, all files of the same piece of
software are guaranteed to contain the same declaration for a
global object or function.
– 2. Should a declaration require updating, only one change to the
header file will need to be made.
A question…
/* C++ code fragment – uses Date class from separate compilation lecture */
Date d1(2, 17, 2006); Date d2;
d2 = d1;
d1.set(2, 20, 2006);
d2.print(); // what gets printed here?
/* Java code fragment – looks nearly identical to above C++ code fragment */
Date d1 = new Date(2, 17, 2006); Date d2;
d2 = d1;
d1.set(2, 20, 2006);
Variables
•
A variable is a symbolic name
assigned to some memory
storage.
•
The size of this storage
depends on the type of the
variable, compiler, and platform.
– e.g., on x86 under Windows, char is 1 byte long and int is 4 byte long.
•
The difference between a
variable and a literal constant is
that a variable is addressable.
354 76 . . . . . . 2000 2004 :x :y
Key distinction: lvalue vs. rvalue
•
A variable has dual roles, depending on where it appears in the
program, it can represent
–
lvalue: the location of the memory storage
–
rvalue: the value in the storage
•
They are so called because a variable represents an lvalue (or
rvalue) if it is written to the left (or right) of an assignment statement.
Thus, the following are invalid statements in C++:
4 = 1;
grade + 10 = new - grade;
x: x+1
Not all languages distinguish rvalues from lvalues
the way that C++ does!
/* C++ code fragment – uses Date class from separate compilation lecture */
Date d1(2, 17, 2006); Date d2;
d2 = d1;
d1.set(2, 20, 2006);
d2.print(); // prints 2006.02.17
/* Java code fragment – looks nearly identical to above C++ code fragment */
Date d1 = new Date(2, 17, 2006); Date d2;
d2 = d1;
d1.set(2, 20, 2006);